Is a trailer aid a good idea for fixing flats? If so, I noticed that there is a "Trailer Aid Plus" that raises the flat tire an extra inch (to 5 and a half inches). Any drawbacks that you know of?

Most of us carry one. Plus a small bottle jack to finish lifting the axle if the aid doesn't get it off the ground far enough to change the flat.
Just be advised---they don't recommend using a jack under the axle.

I have a sunset trail 300bh. On the crossroadroads trailers, at least on mine, the spacing between the axles is quite wide. When you drive up on a standard trailer aid, at least with mine, there is so much droop of the second axle the dependent tire may not clear the ground. I think the trailer aid plus with the additional 1 inch of lift would be the way to go.